Microelectronic device fabrication on a semiconductor wafer requires formation of openings through dielectric thin film layers that are adapted to be filled with metal to provide electrical contact between conductors in different layers. The contact openings typically have an aspect ratio (depth-to-diameter ratio) as high as 40:1. As industry standards progress from 65 nm feature sizes to 45 nm to 32 nm feature sizes, the hole diameter of a contact opening may be reduced to about 630 Å, while the required depth is about 24,700 Å. The opening diameter and the opening-to-opening spacing may be about the same (e.g., roughly 630 Å). Because the aspect ratio of each contact opening is so great, it is essential to have a consistent vertical profile of all the openings, in order to maintain the requisite insulator thickness between adjacent openings. The contact opening location pattern and diameter is defined by an aperture in a patterned photoresist layer that is deposited on the wafer surface prior to formation of the openings. Each photoresist aperture defines a contact opening location and diameter. After photoresist deposition, the contact openings are formed by a plasma etch process that is adapted to etch dielectric material through the apertures in the photoresist layer. The plasma etch process employs a fluorocarbon/fluorohydrocarbon gas that produces two types of species in the plasma: etch species having high fluorine-to-carbon content ratio, and polymer species having a high carbon-to-fluorine content ratio. The polymer species accumulates on exposed surfaces of the sidewall of each opening, which enhances etch selectivity and can reduce the tendency of the etch process to widen the opening beyond the diameter established by the photoresist pattern.
Current plasma etch processes have produced consistently good results at larger feature sizes, e.g., 90 nm. Etch profile is controlled by chamber pressure and by RF bias power. Increasing the RF bias power produces straighter and narrower etch profiles by increasing the ion energy and momentum in the vertical direction. Reducing the chamber pressure can have a similar effect by reducing collisions with ions, thereby reducing the number of ions deflected from their nominal vertical trajectory.
As feature size have been reduced to 45 nm and then to 32 nm, two problems have arisen that in many cases degrade the etch profile sufficiently to threaten device failure. One problem, referred to herein as “bowing”, is manifested by a widened section of the contact opening near the top of the opening. The diameter of the widened section may be as much as twice the desired hole diameter, increasing the likelihood of partial merging of adjacent openings at the widened section. Another problem, referred to herein as “bending”, is manifested by a deflection of the axis of the opening away from true vertical near the bottom of the opening. Such bending has been observed to deflect the center of the opening bottom toward its neighbor by as much as 50% to 100% of the opening diameter. The foregoing problems of bowing and bending have only recently arisen, and coincide with the reduction in device feature size to 45 nm or below, and their cause has remained a mystery. No solution has appeared, although some reduction in bowing or bending has been achieved by reducing the plasma reactor chamber pressure during the plasma etch process and/or increasing RF bias power and ion energy. Such an approach, while reducing the bending or bowing of the etch profile, is problematic in that it reduces the range of chamber pressures (process “window”) over which the plasma etch process may be performed. Similarly, increasing the ion energy by increasing the RF bias power applied to the wafer may have undesired effects upon the etch process, such as a decrease in etch selectivity, photoresist corner faceting, etc. What is needed is a way of preventing bowing and bending without constricting the process window.